Why I cannot add a new method to String.prototype in typescript

Question

I want to add a new method to String.prototype. I tried this.

interface String {
  newMethod(): void
}

String.prototype.newMethod = function() {}

No errors in typescriptlang.org playground. But still show me a error Property 'newMethod' does not exist on type 'String' in my local computer.

I don't know why.

Here is my tsconfig.json

{
  "compilerOptions": {
    "target": "es2015",
    "module": "commonjs",
     "outDir": "./lib",
     "rootDir": "./src",
  }
}

I install `@types/node

I found some example.

// example1: no error
interface String {
  newMethod(): void
}

String.prototype.newMethod = function() {}

// example2: has error
import * as path from 'path'
interface String {
  newMethod(): void
}

String.prototype.newMethod = function() {}

Only import statement added, error occured. So strange. I don't know why?

I am not sure if it can be done here, but I had similar question of "how to change external module interface": stackoverflow.com/questions/46276117/… — libik
– libik, Commented Jan 15, 2018 at 16:02

ganders · Accepted Answer · 2018-01-15 16:44:04Z

4

This is what I did for a "replaceAll" function...

export {};

declare global {
    // tslint:disable-next-line:interface-name
    interface String {
        replaceAll(searchFor: string, replaceWith: string): string;
    }
}

// I hate how the javascript replace function only replaces the first occurrence...
String.prototype.replaceAll = function(this: string, searchFor: string, replaceWith: string) {
    // tslint:disable-next-line:no-invalid-this
    var value = this;
    var index: number = value.indexOf(searchFor);

    while (index > -1) {
        value = value.replace(searchFor, replaceWith);
        index = value.indexOf(searchFor);
    }

    return value;
};

answered Jan 15, 2018 at 16:44

ganders

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1 Comment

XGHeaven Over a year ago

And I also want to know why The example 2 has error? Just because has a import statement?

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